1. Field of the Invention
The present invention relates to digital wireless communication, and particularly to a power control circuit and a power control method used in wireless communications using a CDMA (Code Division Multiple Access) system.
This application is a counterpart of Japanese patent application, Serial Number 281404/2001, filed Sep. 17, 2001, the subject matter of which is incorporated herein by reference.
2. Description of the Related Art
Initially, in the field of CDMA systems, a plurality of mobile stations shared the same frequency band, and signals to other mobile station became interference signals. In particular, attenuation of radio waves is inversely proportional to the square of distance in free space, which means that signals of a base station and mobile stations close to the base station have a significant effect on signals between the base station and mobile stations remote from the base station (near-far problem). In order to solve this near-far problem, it is necessary to make the transmission power of the base station and the mobile station constant, irrespective of the position of the mobile station. To paraphrase the problem, it is necessary to perform control so that SIR (signal-to-interference plus noise power ratio) is constant.
A description will now be given of a conventional power control circuit, using the drawings. FIG. 4 is a block diagram of a conventional power control circuit provided in a mobile station. The conventional power control circuit controls transmission power of a transmission signal from a base station to a mobile station (downlink). The conventional power control circuit comprises an A/D converter 401, a low pass filter (LPF) section 402, N de-spread sections 403, a combining control section 404, a path combining section 405, an SIR measurement section 406, an SIR reference value determination section 407, and an SIR comparison section 408.
Operation of the conventional power control circuit will now be described. The A/D converter 401 converts a receive signal (analog signal) received from a base station into a digital signal. The LPF section 402 removes high frequency components of the converted digital signal, and passes only low frequency components. The de-spread sections 403 carry out de-spread on the receive signals having passed through the LPF section 402, and output a narrowband modulation signal from which spread is removed. Here, a plurality of de-spread sections 403 are provided, corresponding to a plurality of receive signals (paths). The path combining section 405 controls timing and reliability of each path based on the output from the combining control section 404, and creates each path. The SIR measurement section 406 estimates receive SIR based on pilot data 405a of a combined signal.
In the following, estimated receive SIR will be termed SIR measurement value 406a. Also, the SIR reference value determination section 407 determines an SIR reference value based on data 405b of the combined signal. In the following the determined receive SIR reference value will be termed SIR reference value 407a. The SIR reference value is determined using a receive signal error rate etc. The error rate is obtained using receive signal CRC (cyclic redundancy check) data. The SIR comparison section 408 compares the SIR measurement value 406a with the SIR reference value 407a to output control information. Specifically, if the SIR measurement value 406a is greater than the SIR reference value 407a, a control signal to lower the transmit power of the downlink is output. On the other hand, if the SIR measurement value 406a is smaller than the SIR reference value 407a, a control signal to raise the transmit power of the downlink is output.
However, due to shadowing or the like, there are occasions when the receive signal is attenuated suddenly, and the receive signal falls significantly. As a result, there is a problem that communication quality deteriorates, or there is a danger that disconnection will arise.